Lasing dyes are generally defined as substances capable of emitting light when stimulated and typically have, as their lasing media, dye compounds composed of conjugated double bonds. A variety of such dyes are known and commerically available. See generally, Shafer, Dye Lasers (Springer-Verlag 1977) for a discussion of many such dyes. A spectrum of laser radiation, for example, can be achieved (from about 540 nm to about 740 nm) by employing the following dyes in sequence: Rhodamine 590 (6G), Rhodamine 610, Kiton Red, Thodamine 640, Sulforhodamine 640, DCM, Oxazine 720, and Carbazine. Many other lasing dyes are known including some which can extend the radiation spectrum into the infra-red and ultra-violet regions. Dyes are also used for selective absorption as opposed to gain, for example, for saturable absorption in the nonlinear optical systems.
Dye laser systems have become the workhouse for tunable laser applications, such as advanced spectroscopy, and may account for 70 percent of the laboratory laser market. In a typical system, a high intensity source of radiation, such as a nitrogen TEA laser or an argon-ion or krypton-ion laser, is used to optically "pump" the dye solution. The dye will fluoresce at a characteristic wavelength in response to the stimulation. Optical gain is achieved by locating the dye in a resonant cavity. In such systems, a tuning element is also included in the optical resonator to vary the wavelength by a limited amount.
A continuous spectrum is achieved by using a plurality of dyes and fine tuning the system by adjusting the tuning element. A number of commercially available dye laser systems employ a set of dyes encased in cuvettes or cells which are switched manually or under microprocessor control.
A problem common to laser systems employing dyes is that a large percentage of the input energy absorbed from the pump source appears as heat instead of luminescence. The heat leads to optical distortion and ultimately, when the pump focuses on a portion of the dye for too long, the dye compound itself decomposed requiring reactivation or replacement of the lasing medium.
Some commercial dye laser systems now employ free-flowing dye solutions (i.e., using ethylene glycol as a solvent) to transport the lasing dye into and out of the path of the pump beam and optical resonator. For examples of flowing systems and nozzle arrangements see U.S. Pat. No. 3,766,489 issued to Rosenberg in September, 1973; U.S. Pat. No. 3,805,187 issued to Lempicki on Apr. 16, 1974 and U.S. Pat. No 3,984,786 issued to Pike on Oct. 5, 1976. While flowing dye systems can prolong the useful lifetime of a medium, the liquid stream and pump apparatus make changing dyes slow, difficult and possibly dangerous since the dyes are reactive and potentially hazardous to a worker's health. The liquid streams are also limited in ultimate thinness and stability.
There exists a need for simpler, safer dye lasers capable of being tuned over a wide spectrum like the cuvette-switching systems described above but with more resistance to thermal degradation. Such dye lasers should be capable of prolonged use like flowing systems but without the handling and switching problems associated with liquid pumping.